Modern digital oscilloscopes can acquire an electrical signal under test and store digital data corresponding to the acquired signal as a data record. Such oscilloscopes can be used to display one or more waveforms while acquiring the electrical signal under test. In addition, the previously-stored digital data from the data record can be viewed as a waveform on a display of the oscilloscope even though acquisition of the electrical signal under test has stopped. While older oscilloscopes would generally acquire a relatively small data record corresponding to an amount of time that could be displayed, advancements in the areas of memory speed, memory capacity, and processor speed have led to newer oscilloscopes having improved functionality in this area. Consider that most digital oscilloscopes now have the capability of storing a significantly larger data record, which corresponds to more time than can reasonably be displayed at once.
As a result of the larger data records, additional functionality can be made available to the user. For example, most oscilloscopes allow a user to adjust a horizontal or vertical control, thereby changing a position of the waveform on the display. In addition, some oscilloscopes allow a user to “scroll” through the waveform—particularly after the data record has been acquired. However, since the data record now represents much more time than can be displayed at one time, the time interval of user's interest may be short compared to the full acquired record, and scrolling through an entire data record can be impractical. This causes difficulty in finding and analyzing the time interval of interest. Moreover, the full record may contain other events of interest that are different from the one event being displayed.
Some oscilloscopes provide a search feature to flag events of interest in the data record. But due to the rapid advancements in memory capacities, the size of data records continues to expand, which can hinder or otherwise slow down conventional search techniques. Further, such search features require controls or interfaces separate from those with which the users are most familiar.
When viewing a short period of time, the oscilloscope samples the analog signal at a maximum sample rate. The record length of these waveforms is short. For example, when viewing 50 ns with a sample rate of 4 GS/s the record length is 200 samples. Although only a very small number of samples are needed, the oscilloscope acquisition memory is much larger, for example, 40 million samples long. The oscilloscope can be designed to always acquire the full 40 million samples even when the user has only asked to see 50 ns of time. By acquiring this extra data the user can stop the acquisition process and then view portions of the signal outside the original 50 ns time period. This can be useful. However, acquiring a full record takes a considerable amount of time. To acquire 40 million samples at 4 GS/s takes 10 ms. In such a case, the waveform drawing rate is limited to less than 100 waveforms each second.
In order to obtain high waveform throughput, the user can specify a shorter record length, such as a record length of 1,000 samples. However, if the user now stops the acquisition process the instrument has only acquired 1,000 samples and the user is limited to viewing a short period of time even if the oscilloscope has an acquisition memory record length which is considerably longer than 1,000 samples. US patent application Publication 2007/0226406 by Terrance R. Beale et al., discloses a way to write multiple (possibly overlapping) shorter waveforms into a long memory. However, with this technique information between the shorter waveforms may be lost.
What is desired is a way to acquire long records even when a user asks to view a short period of time while also displaying waveforms at rates that are much higher than the acquisition rate of the long record.